The invention relates to an exhaust gas aftertreatment installation for the aftertreatment of an exhaust gas generated by a combustion device, in particular a motor vehicle internal combustion engine, having a nitrogen oxide storage catalytic converter and an SCR catalytic converter with the ability to store ammonia, which is arranged downstream of the nitrogen oxide storage catalytic converter or integrated with the latter in a common catalytic converter unit. The invention also relates to an exhaust gas aftertreatment method which can be carried out using an installation of this type. Installations and methods of this type are used in particular for the aftertreatment or purification of exhaust gases from internal combustion engines operated predominantly in lean-burn mode in motor vehicles.
The use of nitrogen oxide storage catalytic converters, also known as NOx storage catalytic converters or NOx absorber catalytic converters, or NSC for short, is generally known for lowering the levels of nitrogen oxides downstream of internal combustion engines operated in lean-burn mode. Lean operating phases of the internal combustion engine correspond to adsorption phases of the nitrogen oxide storage catalytic converter, in which the nitrogen oxide storage catalytic converter oxidizes nitrogen monoxide (NO) to form nitrogen dioxide (NO2), which it temporarily stores in the form of nitrates. During brief, periodic regeneration or desorption phases, the stored nitrates are removed from the nitrogen oxide storage catalytic converter by the nitrates being converted into nitrogen dioxide and then nitrogen monoxide. The latter is then reduced to form nitrogen by suitable reducing agents.
One known technique used to provide the required reducing agents consists in switching the combustion device, which is operated predominantly in lean-burn mode and the exhaust-gas from which is being aftertreated, to rich-burn mode for a brief period of time, with the result that hydrogen (H2), carbon monoxide (CO) and unburnt hydrocarbons (HC) are present in the exhaust gas as reducing agents. Various specific measures for controlling the air/fuel ratio, also known as the air ratio λ for short, have already been proposed for this purpose, cf. for example the laid-open specifications EP 0 560 991 A1 and DE 196 26 835 A1. The nitrogen oxide reduction may also take place in a downstream catalytic converter known as a deNOx catalytic converter; it is also possible for hydrocarbons to be metered in downstream of the engine in order to provide the reducing agents, cf. for example the laid-open specifications EP 0 540 280 A1 and EP 0 573 672 A1.
A number of problem points need to be borne in mind during this alternating adsorption/desorption operation. For example, considerable quantities of the polluting gas ammonia (NH3) may form through reaction of hydrogen with nitrogen monoxide and/or nitrogen dioxide may form in the regeneration phase, depending on the catalytic converter temperature, the exhaust gas composition and the material composition of the nitrogen oxide storage catalytic converter. When switching from a lean exhaust gas atmosphere to a rich exhaust gas atmosphere, there is the risk of an undesirable breakthrough of nitrogen oxides on account of sudden decomposition of nitrates if suitable quantities of reducing agent are not provided quickly enough. When switching from a rich exhaust gas atmosphere to a lean exhaust gas atmosphere, the nitrogen oxide storage catalytic converter may be heated as a result of exothermic combustion reactions, with the result that nitrates which have already formed can be decomposed again and can predominantly no longer be stored, which can cause undesirable nitrogen oxide slippage. With this NOx storage catalytic converter technology, efficient lowering of the levels of nitrogen oxides is restricted to a relatively narrow temperature range, approximately between 200° C. and 400° C., since at lower temperatures it is difficult to oxidize NO to form NO2, and at higher temperatures the nitrates formed can no longer be stably stored in significant quantities, and the thermodynamic equilibrium between NO and NO2 increasingly shifts toward nitrogen monoxide.
A further problem when using sulfur-containing fuels is what is known as sulfur poisoning of the NOx storage catalytic converter as a result of the accumulation of sulfates, which are more stable than the nitrates and do not decompose during the NOx regeneration phases. Therefore, special desulfating phases at elevated exhaust-gas temperature and with a rich exhaust gas composition are carried out from time to time in order to remove sulfates, cf. for example laid-open specification DE 198 27 195 A1. During desulfating, the polluting gas hydrogen sulfide (H2S) may form; emission of this gas should be avoided. For this purpose, for example in patent DE 100 25 044 C1, it is proposed that secondary air be fed into the exhaust train during the desulfating phases, in order to oxidize the hydrogen sulfide in a downstream oxidation catalytic converter.
Another known exhaust gas aftertreatment method is the process known as selective catalytic reduction, or SCR for short. In this process, a selectively acting reducing agent, typically ammonia, is added to the exhaust gas in order to reduce nitrogen oxides. The ammonia is temporarily stored in a suitable deNOx catalytic converter, also known as an SCR catalytic converter for short, and is used by the latter to catalytically reduce nitrogen oxides (NOx) contained in the exhaust gas to form nitrogen and water. The effectiveness of SCR catalytic converters is highly dependent on the NO/NO2 ratio at low temperatures, with a maximum efficiency at an NO2 level of approx. 50% for temperatures below 200° C. and a greatly reduced efficiency if the NO2 level is lower. At higher temperatures above approx. 400° C., the nitrogen oxide reduction is limited by oxidation of ammonia, and moreover the ammonia storage capacity of the SCR catalytic converter decreases as the temperature rises. The overall result for SCR systems of this type is that the temperature window which is suitable for efficiently lowering the levels of nitrogen oxides is from approximately 250° C. to approximately 550° C. SCR catalytic converters are subject to thermal aging and should not be exposed to temperatures of over approximately 700° C. to 750° C. At low temperatures, SCR catalytic converters may also temporarily store unburnt hydrocarbons, and even when the exhaust gas composition is rich, given a suitable design, they may oxidize hydrocarbons, in particular if they contain vanadium oxide (V2O5) as a catalytic material.
To provide the ammonia in the exhaust gas, it is known to introduce urea, from which ammonia is formed through hydrolysis or thermolysis. One problem in this context, in particular for vehicle applications, is that a corresponding supply of urea has to be carried around. Therefore, as an alternative, internal generation of ammonia has already been proposed. For this purpose, it is proposed in laid-open specifications WO 97/17532 A1 and DE 199 49 046 A1 that an ammonia-generating catalytic converter and, downstream of the latter, an ammonia storage and nitrogen oxide reduction catalytic converter be provided, preferably with the addition of an oxygen storage catalytic converter between the two catalytic converters mentioned above or downstream of the ammonia storage catalytic converter and the nitrogen oxide reduction catalytic converter.
Laid-open specification EP 0 878 609 A1 has described an exhaust gas aftertreatment installation of the generic type, in which an NOx storage catalytic converter and, downstream of the latter, an SCR catalytic converter are arranged in the exhaust train. As an alternative or in addition to a three-way catalytic converter, the NOx storage catalytic converter can be designed to form ammonia when the engine is briefly operated under rich-burn conditions, which is realized by an afterinjection of fuel into at least some of the engine combustion chambers. With this type of internal generation of ammonia, there is a risk of the quantity of ammonia which is generated exceeding the ammonia storage capacity of the SCR catalytic converter, resulting in undesirable ammonia slippage.
The exhaust gas aftertreatment measures described above do not lower the levels of particles, in particular of carbon particulates. It is known to use particulate filters to do this. Carbon particulates which have collected in the particulate filter can be burnt off at elevated temperature in the presence of oxygen. A standard measure for heating the particulate filter consists in introducing fuel into the exhaust gas, for example by an afterinjection, and burning this fuel in an oxidation catalytic converter connected upstream of the particulate filter. What are known as CRT systems with continuous particulate filter regeneration are also known, cf. for example U.S. Pat. No. 4,902,487. In these systems, nitrogen dioxide is formed from NO at the oxidation catalytic converter and oxidizes the carbon particulates which have collected in the particulate filter.
To lower the levels of particulate and nitrogen oxides, laid-open specification DE 199 21 974 A1 describes an exhaust gas aftertreatment installation having an oxidation catalytic converter, a downstream NOx storage catalytic converter and a particulate filter downstream of the NOx storage catalytic converter or between the oxidation catalytic converter and the NOx storage catalytic converter. In the case of the former arrangement, the oxidation catalytic converter promotes the function of the NOx storage catalytic converter through the formation of NO2, but it is not possible to achieve a CRT effect for the particulate filter, since the nitrogen oxides are already being reduced to nitrogen upstream of the particulate filter. If the oxidation catalytic converter is used to heat the particulate filter by combustion of afterinjected fuel, this arrangement results in a high thermal loading of the NOx storage catalytic converter and a relatively high fuel consumption. In the other arrangement described, the exhaust gas upstream of the NOx storage catalytic converter predominantly contains NO and only a small amount of NO2, since the latter is converted into NO by the CRT effect, with the result that the storage characteristics of the NOx storage catalytic converter deteriorate. If the oxidation catalytic converter in this arrangement is used for desulfating-heating of the NOx storage catalytic converter through combustion of afterinjected fuel, the high heat capacity and the heat transfer in the intervening exhaust train section means that a relatively high temperature has to be reached at the oxidation catalytic converter, which can lead to thermal aging effects in the latter.
The invention is based on the technical problem of providing an exhaust gas aftertreatment installation of the type described in the introduction and an associated exhaust gas aftertreatment method with which as many of the following demands as possible can be satisfied to a high degree with relatively little outlay: effective reduction of nitrogen oxides in a wide temperature range, no need for an additional reducing agent operating medium, avoidance of ammonia and hydrogen sulfide emissions, minimal particulate emissions, particulate oxidation through NO2 reaction, minimal CO and HC emissions, relatively low thermal loading of all the components used to purify the exhaust gas, minimal increased fuel consumption and a low demand for installation space.
The invention solves this problem by providing an exhaust gas aftertreatment installation for the aftertreatment of an exhaust gas generated by a combustion device, in particular a motor vehicle internal combustion engine, having a nitrogen oxide storage catalytic converter and an SCR catalytic converter with the ability to store ammonia, which is arranged downstream of the nitrogen oxide storage catalytic converter or integrated with the latter in a common catalytic converter unit.
The invention also solves this problem by providing a method for the aftertreatment of an exhaust gas generated by a combustion device, in particular a motor vehicle internal combustion engine, in which nitrogen oxides contained in the exhaust gas are temporarily stored in a nitrogen oxide storage catalytic converter during adsorption operating phases of the latter and are released again from the nitrogen oxide storage catalytic converter during regeneration operating phases of the latter, with ammonia being generated, and ammonia which is generated is temporarily stored in a downstream SCR catalytic converter and is used for nitrogen oxide reduction, characterized in that the recorded nitrogen oxide content of the exhaust gas downstream of the SCR catalytic converter and/or downstream of the nitrogen oxide storage catalytic converter and/or the ammonia loading of the SCR catalytic converter is used as a criterion for the instant at which a regeneration operating phase is triggered for the nitrogen oxide storage catalytic converter can further include the following step wherein nitrogen oxides contained in the exhaust gas are temporarily stored in a nitrogen oxide storage catalytic converter during adsorption operating phases of the latter and are released again from the nitrogen oxide storage catalytic converter during regeneration operating phases of the latter, with ammonia being generated, and ammonia which is generated is temporarily stored in a downstream SCR catalytic converter and is used for nitrogen oxide reduction, characterized in that a desired ammonia generation quantity which is to be generated during a current regeneration operating phase of the nitrogen oxide storage catalytic converter is determined, and the subsequent regeneration operating phase is carried out as a function of the desired ammonia generation quantity determined.
In addition to a nitrogen oxide storage catalytic converter and an SCR catalytic converter connected downstream of it, the exhaust gas aftertreatment installation additionally includes a particulate filter and/or an NO2-producing catalytic converter arranged upstream of the SCR catalytic converter.
The nitrogen oxide storage catalytic converter allows the levels of nitrogen oxides to be effectively lowered, in particular even in the case of lean-burn internal combustion engines. By virtue of its NH3 storage capacity, the downstream SCR catalytic converter prevents undesired emission of ammonia generated by the NOx storage catalytic converter. At the same time, the SCR catalytic converter is able to use stored ammonia to reduce any nitrogen oxide which may still be present in the exhaust gas downstream of the NOx storage catalytic converter, with the ammonia being oxidized at the same time. This effect can be exploited to deliberately form ammonia at the nitrogen oxide storage catalytic converter, in order for this ammonia to be used as reducing agent in the SCR catalytic converter. As a result, the effective lowering of the levels of nitrogen oxides can be maintained even during periods in which the temperature of the NOx storage catalytic converter is temporarily outside its range in which it has a conversion activity, for example as a result of the exothermicity following a transition from a lean exhaust gas composition to a rich exhaust gas composition.
If nitrogen oxides are still present in the exhaust gas downstream of the NOx storage catalytic converter, these nitrogen oxides are generally in the form of NO; the NO2 content is much lower, amounting, for example, to at most 20%. However, at low temperatures below 300° C., the efficiency of the SCR catalytic converter is highest approximately at an NO2 level of 50%, and significantly reduced where the NO2 level is lower. To increase the efficiency of the SCR catalytic converter, the NO2-producing catalytic converter may be connected upstream of it. This catalytic converter may have a relatively small volume and may have a coating which, inter alia, contains a precious metal (e.g. platinum) and is able to increase the NO2 level in the NOx emission to at least approximately 50% at least in a temperature range from approximately 200° C. to 350° C. One particular feature of the coating of the NO2-producing catalytic converter is its property of deliberately not oxidizing ammonia which is generated in the NOx storage catalytic converter when operating at λ<1, but rather allowing this ammonia to pass through in unchanged form. This can be achieved, for example, by the coating not containing an oxygen-storing component.
In a corresponding control unit, which can also be used, for example, to control the combustion device, such as an internal combustion engine, it is preferable to implement functions which decide on the need for and feasibility of targeted generation of NH3 and preset the operating parameters, in particular the duration and extent of enrichment during NSC regeneration, in a suitable way. The formation of NH3 can typically be boosted by a lower air ratio and a longer regeneration time, provided that the temperature of the NOx storage catalytic converter is in the range in which NH3 formation is possible. Furthermore, operation of the combustion device during NSC regeneration can be set in a manner known per se such that high untreated NOx emissions from the combustion device result, and as a result the formation of NH3 at the NOx storage catalytic converter is boosted further.
It has been found that the SCR catalytic converter can also be used to avoid H2S emission, produced, for example, during desulfating. Tests have shown that an SCR catalytic converter, on account of its specific properties, can oxidize hydrogen sulfide formed during the desulfating to form SO2 even at a rich exhaust gas composition (λ<1). This makes it possible to avoid unpleasant odor pollution.
As a further particular feature, SCR catalytic converters, provided that they contain vanadium oxide, can even oxidize unburnt hydrocarbons (HC) during rich conditions (λ<1). This makes it possible to reduce the extent to which reducing agent breaks through during NSC regeneration. In particular, by way of example, it is possible to lower the emission levels of possibly carcinogenic hydrocarbons, such as benzene, toluene, ethylbenzene and xylene, which can form under rich conditions at the NOx storage catalytic converter. On account of its ability to store hydrocarbons even at low temperatures, the SCR catalytic converter can additionally make a contribution to lowering the levels of HC emissions following a cold start. The HC which have been stored at low temperatures are released again at higher temperatures and can be oxidized at the SCR catalytic converter or a downstream oxidation catalytic converter.
A particulate filter can be used for lowering the levels of particulates downstream of the engine. This particulate filter highly efficiently retains the particulates emitted. As is customary, the particulates which have collected can be burnt off at regular intervals by increasing the temperature to over 600° C. If the exhaust gas reaching the particulate filter contains NO2, particulate oxidation also takes place even in the temperature range between approximately 250° C. and 400° C. as a result of reaction with NO2 (CRT effect). The particulate filter may generally be catalytically coated, in which case the coating may contain components such as a precious metal (e.g. platinum) and a washcoat.
The maximum thermal loading of the individual components can be adapted to the specific requirements by using a suitable arrangement of the components. Moreover, by using a suitable arrangement it is possible to ensure that the temperatures of the individual components are within a range which is expedient for the corresponding function when the vehicle is driving. The rich-burn operation which is required for the regeneration of the NOx storage catalytic converter can be realized by engine-internal measures or additional introduction of reducing agent (e.g. fuel or hydrogen) downstream of the engine.
The heating of the NOx storage catalytic converter for desulfating purposes and of the particulate filter for thermal regeneration purposes can be effected by engine-internal measures, including afterinjection of fuel. In addition to the deliberately increased exhaust gas temperature, incompletely burnt hydrocarbons which remain in the exhaust gas lead to additional exothermicity at a catalytic converter optionally arranged close to the engine, with the result that the exhaust gas temperature is increased further. In addition or as an alternative, it is also possible for reducing agent (e.g. fuel or hydrogen) to be supplied in the exhaust train directly upstream of the component(s) to be heated or upstream of an oxidation catalytic converter connected upstream of these components. This has the advantage that the heat losses involved in heating further upstream components and heat losses resulting from cooling in the exhaust pipe are reduced. This restricts the outlay on energy and therefore the increased fuel consumption for the heating to a minimum. A further advantage is that in this way further upstream components are not subjected to high exhaust gas temperatures, and consequently the thermal aging of these components can be restricted to a minimum. Moreover, this prevents further upstream components, e.g. an upstream NOx storage catalytic converter, from leaving the temperature window required for high efficiency as a result of the heating.
In the case of a catalytically coated particulate filter, a further advantage is that the conversion of fuel, on account of the high heat capacity of the particulate filter, continues to be possible even, for example, after prolonged overrun phases of the internal combustion engine with a low exhaust gas temperature. By contrast, when using a conventional catalytic converter, there is a risk that, on account of the low heat capacity, the temperature will drop below the light-off temperature under comparable conditions, meaning that catalytic conversion of the hydrocarbons will no longer be possible. In general terms, other heating methods are possible as an alternative to supplying reducing agent (e.g. hydrogen or fuel) upstream of a catalytic converter. These other methods are not expressly mentioned below but can be used instead of supplying reducing agent downstream of the engine. In this context, by way of example, mention may be made of an electrically heated catalytic converter, electrical heating of the particulate filter or the use of a burner as standard conventional measures.
The supply of reducing agent downstream of the engine and upstream of the NOx storage catalytic converter can also be utilized to set rich conditions for NSC regeneration when the engine is operating with lean exhaust gas. This is preferably implemented when the engine is operating between λ=1.0 and λ=1.2, since otherwise the quantity of reducing agent to be supplied is too great. This results in the advantage that high untreated NOx emission usually occurs in the range between λ=1.0 and λ=1.2, whereas the level of untreated NOx emissions is considerably lower at air ratios of λ<1. Consequently, this method can be used to achieve a high level of NOx emission and therefore extensive formation of NH3 during NSC regeneration.
To avoid high emissions of CO and HC during NSC regeneration operations with λ<1, if necessary secondary air can be blown in ahead of a downstream oxidation catalytic converter. The secondary air can, for example, be provided by an electrically driven secondary air pump or a compressor or, in the case of supercharged engines can be removed downstream of the compressor.
It is possible to considerably reduce the overall space taken up by optionally combining or integrating two of the abovementioned functionalities in one component, for example, by applying a catalytic coating to a particulate filter.
In further configurations, the exhaust gas aftertreatment installation contains one or more NOx sensors downstream of the NOx storage catalytic converter and/or the SCR catalytic converter, and/or means for recording the temperature of one or more of the exhaust gas purification components and/or means for recording the NH3 loading of the SCR catalytic converter.
Various possible implementations are possible for the order in which the NOx storage catalytic converter, SCR catalytic converter, particulate filter and NO2-producing catalytic converter are arranged in the exhaust train, each of these implementations having specific properties and advantages; the options include multi-flow arrangements.
The basic method described above allows relatively accurate, model-based control of the time at which a regeneration of the nitrogen oxide storage catalytic converter is initiated.
The second method described above allows targeted control of the generation of ammonia during a respective regeneration phase of the nitrogen oxide storage catalytic converter taking account of the current conditions, in particular with regard to the temperatures of the ammonia-generating NOx storage catalytic converter and of the SCR catalytic converter and/or the ammonia loading of the SCR catalytic converter; a variable quantity of ammonia which is to be generated during the current regeneration phase of the NOx storage catalytic converter can be predetermined as a function of the operating state which has been ascertained.
In one configuration of this method, the results of the determination as to whether the exhaust gas air ratio downstream of the NOx storage catalytic converter has dropped below a threshold value, which is dependent on the quantity of ammonia which it is desired to form, can be used as a criterion for terminating a regeneration phase of the NOx storage catalytic converter.
In a further configuration of the method according to the invention, an external supply of reducing agent, e.g. downstream of the engine, into the exhaust train can be provided for during the NSC regeneration, in order for the combustion device also to be operated under lean-burn conditions during this period, resulting in high untreated NOx emissions from the combustion device.
In a further configuration of the method, it is possible to provide for secondary air to be fed into the exhaust train at a suitable location during the rich-burn operating phases, in order to oxidize any NH3, H2S, CO and HC which may be present during these operating phases and thereby to prevent corresponding emissions of pollutants.
Advantageous embodiments of the invention are illustrated in the drawings and described below.